The present invention relates to the method, device and catalyst to clean exhaust gas discharged from an internal combustion engine including a car, and particularly to the method, device and catalyst to clean exhaust gas discharged from an internal combustion engine permitting lean burning of fuels and from a car equipped with said engine.
According to a heretofore known method, an exhaust gas cleaning catalyst is placed in the exhaust gas flow path of the internal combustion engine, and exhaust gas having an air-fuel ratio higher than theoretical air-fuel ratio (hereinafter referred to as “oxidation atmosphere”) and exhaust gas having an air-fuel ratio equal to or smaller than theoretical air-fuel ratio (hereinafter referred to as “reduction atmosphere”) are alternately made to contact said catalyst, thereby removing nitrogen oxides in exhaust gas. This method is disclosed, for example, in the Official Gazette of Japanese Patent Laid-Open NO. 212933/1998.
According to the Official Gazette of Japanese Patent Laid-Open NO. 327617/1997, a catalyst used in this type of exhaust gas cleaning method includes an alkaline earth metal and titanium. A catalyst containing titanium in the form of amorphous material is disclosed in said Gazette. The Official Gazette of Japanese Patent Laid-Open NO. 109032/1998 discloses a catalyst containing alkaline earth metal and titanium, wherein part of these materials take the form of composite oxides. The Official Gazettes of Japanese Patent Laid-Open NO. 327617/1997 and NO. 109032/1998 disclose that SOx contained in exhaust gas becomes difficult for alkaline earth metal to capture, and this makes it possible to suppress poisoning by SOx.
Inventions described in the Official Gazettes of Japanese Patent Laid-Open NO. 327617/1997 and NO. 109032/1998 are mainly intended to prevent alkaline earth metal as an NOx capturing agent from being poisoned by SOx. However, alkaline earth metal is not always immune to poisoning by SOx; it is subjected to poisoning by SOx if a long-term operation is performed in the oxidation atmosphere, and NOx removing performances are deteriorated.